U.S. patent number 8,779,046 [Application Number 12/824,451] was granted by the patent office on 2014-07-15 for polymer composition with uniformly distributed nano-sized inorganic particles.
This patent grant is currently assigned to Dupont Mitsui Fluorochemicals Co Ltd. The grantee listed for this patent is Jeong Chang Lee, Pham H. Nam. Invention is credited to Jeong Chang Lee, Pham H. Nam.
United States Patent |
8,779,046 |
Nam , et al. |
July 15, 2014 |
Polymer composition with uniformly distributed nano-sized inorganic
particles
Abstract
A polymer composition is disclosed wherein inorganic particles
are uniformly dispersed at the nano level in a polymer without
having the inorganic particles being surface-treated. Further
disclosed is a method for manufacturing a polymer composition
wherein a co-aggregate which is obtained by uniformly mixing
polymer dispersion with an inorganic particle colloidal solution
and co-aggregating the polymer primary particles and inorganic
particles which are heterogeneous particles, is separated from the
solvent and dried so that the inorganic particles are uniformly
dispersed at the nano level in the polymer.
Inventors: |
Nam; Pham H. (Shizuoka,
JP), Lee; Jeong Chang (Shizuoka, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Nam; Pham H.
Lee; Jeong Chang |
Shizuoka
Shizuoka |
N/A
N/A |
JP
JP |
|
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Assignee: |
Dupont Mitsui Fluorochemicals Co
Ltd (N/A)
|
Family
ID: |
37904075 |
Appl.
No.: |
12/824,451 |
Filed: |
June 28, 2010 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20100261809 A1 |
Oct 14, 2010 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11540871 |
Sep 29, 2006 |
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Foreign Application Priority Data
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Sep 30, 2005 [JP] |
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2005-287314 |
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Current U.S.
Class: |
524/410; 524/492;
523/335 |
Current CPC
Class: |
C08K
3/01 (20180101); C08K 3/2279 (20130101); C08K
3/22 (20130101); C08K 3/36 (20130101); C08K
3/01 (20180101); C08L 27/12 (20130101); C08K
3/22 (20130101); C08L 27/12 (20130101); C08L
27/12 (20130101); C08K 3/2279 (20130101); C08L
27/12 (20130101); C08K 3/22 (20130101); C08K
3/2279 (20130101); C08L 27/12 (20130101); C08K
3/36 (20130101); C08L 27/12 (20130101); C08K
3/01 (20180101); C08L 27/12 (20130101) |
Current International
Class: |
C08C
1/14 (20060101); C08K 3/36 (20060101) |
Field of
Search: |
;248/403 ;260/29.6,132
;427/212 ;428/336,402,404,407,421,422 ;523/333,335
;524/265.269,333,430,432,442,444,445,462,463,492,493,495,544,410
;525/330 ;526/245,246,247,255 ;528/401 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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04-202329 |
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Jul 1992 |
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JP |
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03078315 |
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Sep 2003 |
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WO |
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2005084955 |
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Sep 2005 |
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WO |
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Other References
Abstract of JP 408120210A, Published May 14, 1996. cited by
examiner .
Kunio Furusawa et al, "Heterocoagulation Behaviour of Polymer
Latices With Spherical Silica", Colloids and Surfaces, 63 (1992)
103-111. cited by applicant .
Hirose, M. et al, "Organic/Inorganic Nanocomposite Obtained by
Crushing and Dispersing Porous Silica", Edited by Powder
Engineering Association: Powder Engineering Handbook--Second
Version, p. 291-294 (1986). cited by applicant .
Homola, A. et al, "Preparation and Characterization of Amphoteric
Polystyrene Latices", Journal of Colloid and Interface Science,
vol. 59, No. 1, Mar. 15, 1977. cited by applicant .
Wang, H. et al, Polyimide/Silica/Titania Nanohybrides Via a Novel
Non-Hydrolytic Sol-Gel Route, Composites Part A: Applied Science
and Manufacturing, Elsevier Science Publishers B.V., Jul. 2005, pp.
909-914, vol. 36, No. 7, Amsterdam, NL. cited by applicant .
Gellermann C. et al, "Functionalisierte Nanopartikel--Nur Als
Fuellstoff Geeignet?", Gummi, Fasern, Kunststoffe. Internationale
Fachzeitschrift Fur Die Polymer-Verarbeitung, Gentner Verlag,
Stuttgart, DE, vol. 53, No. 10, Oct. 2000, pp. 712-717. cited by
applicant.
|
Primary Examiner: Choi; Ling
Assistant Examiner: Wang; Chun-Cheng
Attorney, Agent or Firm: Palmer; Keith W.
Claims
What is claimed is:
1. A method for making a polymer composition, comprising mixing
aqueous polymer dispersion comprising polymer primary particles
with an aqueous colloidal solution of spherical inorganic particles
having an average diameter of 10 to 300 nm, said inorganic
particles being hydrophilic, stabilized, not surface-treated, and
stably dispersed in said solution, the ratio
(D.sub.inorganic/D.sub.polymer) of the average particle diameter
(D.sub.inorganic) of said inorganic particles to the average
primary particle diameter (D.sub.polymer) of said polymer primary
particles being 0.1 to 2.0, wherein said inorganic particles are
present in 0.1 to 40 wt % based on the combined weight of said
polymer and said inorganic particles, coagulating the resultant
mixture by the addition of electrolyte, wherein said electrolyte is
in the form of an aqueous solution and the amount of said
electrolyte is from 0.02 to 10 weight percent based on the combined
weight of said electrolyte and said polymer, to make a co-aggregate
of the polymer primary particles with said inorganic particles,
separating said co-aggregate, and drying said co-aggregate.
2. The method of claim 1, wherein said inorganic particles of said
colloidal solution are selected from at least one of the group
consisting of silicon oxide, titanium oxide, aluminum oxide, and
zinc antimonate.
3. The method of claim 1, wherein the polymer of said polymer
dispersion is a polymer or copolymer of monomers which are selected
from the group consisting of tetrafluoroethylene,
hexafluoropropylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl
ether), vinylidene fluoride and vinyl fluoride, or a copolymer of
ethylene or propylene with at least one of the monomers selected
from the group consisting of tetrafluoroethylene,
hexafluoropropylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl
ether), vinylidene fluoride and vinyl fluoride.
4. The method of claim 1, further comprising melt processing said
composition as said co-aggregate, or as granules or pellets of said
co-aggregate.
5. The method of claim 1, further comprising compression molding
said composition as said co-aggregate, or as granules of said
co-aggregate.
6. The method of claim 1 wherein the ratio (V.sub.0.1/V.sub.1) of
the melt viscosity (V.sub.0.1) at 0.1 rad/sec to the melt viscosity
(V.sub.1) at 1 rad/sec of said polymer composition is 1.4 or
greater, said melt viscosities being measured at 340.degree. C. by
the parallel-plate mode of a dynamic viscosity and elasticity
measuring device.
7. The method of claim 1, wherein the storage elastic modulus at
200.degree. C. of said polymer composition is greater than 1.7
times than that of said polymer itself.
Description
FIELD OF INVENTION
The present invention relates to a polymer composition wherein
inorganic particles are uniformly dispersed at the nano level in a
polymer and to a method for manufacturing said polymer
composition.
BACKGROUND OF THE INVENTION
A conventional means for improving the properties such as
mechanical strength, dimensional stability, and compression creep
resistance of polymers has been to combine a filler with polymer.
However, the uniformity with which filler distributed in the
polymer is not completely specified.
Recently, methods have been developed to improve the mechanical
strength, heat deformation temperature and dimensional stability of
polymer by direct melt blending of nano particles such as inorganic
nano particles into the polymer. However, when inorganic particles
are melt-mixed with polymer, the mutual cohesive force of the
particles is found to increase as particle diameter is decreased
and the inorganic particles tend to aggregate, that is, the
inorganic particles cluster together, especially at when the
particles are at the nano level in size, i.e. about 1 to 1000 nm in
diameter. Therefore, even when the nano particles are directly
melt-mixed with the polymer, it is extremely difficult to disperse
the particles at the nano level in the polymer (Powder Body
Engineering Handbook, 2nd Edition, p. 291-294, 1983, as reported in
the Proceedings of the 47.sup.th Meeting of the Japan Society of
Materials Science, Kyoto, Oct. 29-30, 2003, pp. 150-151).
One approach to overcoming the problems of the above described
direct melt-mixing method, is a solution-mixing method wherein a
colloidal solution of stably dispersed inorganic particles is mixed
with a functionalized polymer dissolved in a liquid. For example,
U.S. Pat. No. RE37,022 proposes a composition (coating agent)
wherein perfluoropolymer is dissolved in an organosol wherein
inorganic particles with an average particle diameter of 1000 nm or
less, treated with fluorine-containing surfactant, are dispersed in
a fluorinated solvent having no hydrogen (that is, the solvent has
no hydrogen atoms bonded to the multivalent atoms of the solvent
molecule) or a solvent made by mixing said solvent with a
fluorinated solvent that contains hydrogen. The use of
functionalized fluoropolymer and fluorinated solvents make this an
expensive and inconvenient approach, suitable only for specialized
applications.
U.S. Pat. No. 6,350,806 is directed to water-based paint, which is
cured at up to 300.degree. C., made of aqueous fluoropolymer
dispersion which is added to aqueous-emulsified acrylate and
methacrylate monomer, which are then polymerized. To the resulting
polymer dispersion is mixed dry colloidal silica that is been
treated with organoalkoxysilane. In the absence of the
organoalkoxysilane, the silica is not stably mixed and the paint
lacks storage stability. The distribution of the treated silica
particles in the dried paint coating is not disclosed. Being a
paint, the composition is not suitable for compression molded,
extrusion molded, or injection molded articles. In view of the
substantial acrylic content, 30 parts acrylate to 100 parts
fluoropolymer, the resulting polymer composition lacks the thermal
properties and oxidative resistance characteristic of
fluoropolymers.
Another approach to the above described direct melt-mixing method
is reported in Colloid and Surfaces, vol. 63, p. 103-111, 1992
wherein it is disclosed that aggregate is created from the solution
made from mixing heterogeneous particles, wherein a colloidal
silica to which potassium chloride is added so that the pH value is
5.6, is mixed with polystyrene emulsion in which the polystyrene is
a copolymer that includes comonomers that provide acid and base
functionality whereby the polymer is amphoteric. The silica and
polystyrene particles have opposite electrical charge and thus form
an unstable mixture, wherein slight mixing causes the particles to
form heterocoagulates. This reference requires that the ratio of
the diameter of the silica primary particles to that of the polymer
primary particles be 3 or more to obtain the proper aggregate
composed of a relatively large silica core and small
amphoteric-modified polystyrene particles clustered around the
core. These aggregates are disclosed to be useful as functional
particles in industrial fields.
U.S. Patent Application Publication No. 2005/0123739 discloses
dispersing dry mesoporous hydrophobic-modified fused silica into
polytetrafluoroethylene dispersion, which is then coagulated, and
the liquid drained, and the coagulate dried at 130.degree. C.,
followed by calendering into sheet form, and sintering to improve
electrical properties as printed circuit substrates.
Japanese published examined application No. Hei 7-64936 proposes a
method for obtaining a powder with an average particle diameter of
3 mm wherein a suspension of silicon carbide particles with an
average particle diameter of 4000 nm that has been surface-treated
with an aminosilane group surfactant, is added to a fluoropolymer
dispersion. Then nitric acid is added to the mixture to break the
emulsion and after that, trichlorotrifluoroethane is added to the
mixture to coagulate and granulate the particles thereby obtaining
an powder with an average particle diameter of 3 mm.
None of the above-mentioned teachings solve the problem of
providing a molded article of filled polymer where the filler is
nano-sized and is uniformly dispersed as such in the polymer.
SUMMARY OF THE INVENTION
The present invention solves this problem by the method of
manufacturing the polymer composition, comprising mixing aqueous
polymer dispersion comprising polymer primary particles with
aqueous colloidal solution of inorganic particles having an average
particle diameter of 1 to 1000 nm, coagulating the resultant
mixture to make a co-aggregate of the particles, separating said
co-aggregate from the aqueous media of said solution, and drying
the coaggregate.
In a preferred method, a polymer dispersion is formed, wherein the
polymer primary particles are surrounded by a surfactant (which may
hereinafter be called emulsifying agent) and stably dispersed in
the aqueous medium in the course of emulsion polymerization, is
mixed with an aqueous colloidal solution (which may hereinafter be
called an inorganic particle sol). Then, after the polymer primary
particles are uniformly mixed with the inorganic particles in the
mixed aqueous media, the resultant mixture is coagulated so that
the uniformly mixed polymer primary particles and inorganic
particles are solidified, i.e., co-aggregated, to distinguish from
aggregates of primary polymer particles with each other, and
aggregates of inorganic particles with each other. Then, by
separating the co-aggregate from the aqueous phase and drying,
dried co-aggregate of the inorganic particles dispersed at the nano
level in the polymer is obtained. The mixture of the inorganic
particle sol with the stably dispersed polymer primary particles
results in the inorganic particles also being stably dispersed at
the nano level in admixture with the polymer primary particles.
After the polymer primary particles and inorganic particles are
uniformly mixed, the resulting mixture is subject to coagulation by
such techniques a vigorous mechanical mixing (a strong shearing
force), by adding electrolyte to the mixture, or by freezing the
mixture (dispersion). In this way, the stability of the dispersed
admixture of the polymer particles and of the inorganic particles
is decreased thereby coagulating the particles together. As a
result, the uniformly mixed polymer primary particles and inorganic
particles are solidified. Then, by separating the co-aggregate from
the aqueous medium and drying the co-aggregate, the polymer
composition is obtained wherein the inorganic particles are
uniformly dispersed at the nano level with the primary particles of
the polymer. On melting, compression molding, or sintering of the
polymer, a composition is obtained wherein inorganic particles are
uniformly dispersed in the polymer at the nano level, i.e. the
inorganic particles are of nano dimensions (1000 nm and smaller in
particle size) in the polymer matrix. Therefore, the present
invention can be used in a variety of fields that require inorganic
particles to be uniformly dispersed at the nano level in a polymer
matrix.
Another preferred embodiment of the present invention is the
polymer composition derived from this method wherein inorganic
particles are uniformly dispersed at the nano level in the polymer.
By derived is meant directly obtained from the method, i.e. the
co-aggregates, or indirectly obtained by processing of the
co-aggregates, to make e.g. granules, pellets, or molded articles
such as by compression molding or melt mixing fabrication.
A preferred embodiment of the present invention is the granulated
powder which is obtained by granulating the polymer
composition.
A preferred embodiment of the present invention is the pellet which
can be obtained by melt-mixing in the course of extruding the
polymer composition or of the granulated powder of the polymer
composition.
Another preferred embodiment of the present invention is the
composition obtained by melt-mixing the polymer composition derived
by mixing polymer dispersion comprising polymer primary particles
with the colloidal solution of inorganic articles, coagulating the
resultant mixture to make a co-aggregate of the polymer primary
articles with said inorganic particles, separating said
co-aggregate, and drying said co-aggregate. The co-aggregate can be
melt processed as such or after granulation or pelletization
thereof. The melt mixing can also be applied to the co-aggregate,
granules, or pellets thereof. Compression molding is preferably
carried out with the co-aggregates or granules thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph of the measurement of the zero shear rate
viscosity of polymer compositions containing 20 wt % of silica.
FIG. 2 is a graph of the measurement of the zero shear rate
viscosity of the polymer compositions containing 10 wt % and 20 wt
% silica from PL-7 silica sol.
FIG. 3 is an electron microscope picture of the cross section
obtained by fracture of a compression molding of the polymer
composition sample of Example 1.
FIG. 4 is an electron microscope picture of the cross section
obtained by fracture of a compression molding of the polymer
composition sample of Example 4.
FIG. 5 is an electron microscope picture of the surface of the
dried-powder co-aggregate of Example 4.
FIG. 6 is an electron microscope picture of the cross section
obtained by fracture of a compression molding of the polymer
composition sample of Example 7.
FIG. 7 is an electron microscope picture of the surface a coating
of the polymer composition sample of in Comparative Example 2.
FIG. 8 is an electron microscope picture of the cross section
obtained by fracture of a compression molding of the polymer
composition sample of Comparative Example 3.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a polymer composition wherein
inorganic particles are uniformly dispersed at the nano level in
the polymer and a method for manufacturing said polymer
composition. The polymer dispersion used in the present invention
is not limited to a specific dispersion and any polymer dispersion
can be used. Fluoropolymers are the preferred polymers. Examples of
fluoropolymer dispersions include polymer or copolymer of the
monomers selected from tetrafluoroethylene (TFE),
chlorotrifluoroethylene (CTFE), trifluoroethylene,
hexafluoropropylene (HFP), perfluoro(alkyl vinyl ether) (PAVE),
which includes perfluoro(methyl vinyl ether) (PMVE),
perfluoro(ethyl vinyl ether) (PEVE), perfluoro(propyl vinyl ether)
(PPVE), vinylidene fluoride (VdF or VF2)) and vinyl fluoride (VF),
or a copolymers of the above monomers with ethylene or
propylene.
Examples of the fluoropolymer dispersion include
polytetrafluoroethylene (hereinafter called PTFE), TFE/PAVE
copolymer (which hereinafter may be called PFA, a species of which
is sometimes called PMA if PMVE is among the monomers used),
tetrafluoroethylene/ethylene copolymer (ETFE), polyvinylidene
fluoride (PVDF), polychlorotrifluoroethylene (PCTFE),
chlorotrifluoroethylene/ethylene copolymer (ECTFE), TFE/VdF
copolymer, TFE/VF copolymer, TFE/HFP/VF copolymer, HFP/VdF
copolymer, VdF/CTFE copolymer, TFE/VdF/CTFE copolymer and
TFE/HFP/VdF copolymer.
Among them, in the copolymer of tetrafluoroethylene and
perfluoro(alkyl vinyl ether), the number of carbons of alkyl group
is preferably 1 to 5, or more preferably, 1 to 3. It is preferable
that the dispersion of the above described polymers and copolymers
is manufactured by emulsion polymerization.
According to the present invention, polymer dispersion wherein the
polymer primary particles are surrounded by a surfactant and stably
dispersed in the dispersing liquid in the course of emulsion
polymerization, is mixed with a colloidal solution wherein
inorganic particles are stably dispersed and the polymer primary
particles are thereby uniformly mixed with the inorganic particles.
This mixture is stable since while the polymer primary particles
and the inorganic particles are interdispersed within the aqueous
medium derived from the aqueous dispersion and the colloid, these
particles are not attracted to one another to cause aggregation.
The mixing and coagulation steps, the latter causing
co-aggregation, are sequentially, not simultaneously carried out.
To cause coagulation, the stability of the colloid solution is
decreased, such as by shearing or other means disclosed herein.
Therefore, it is possible to obtain a polymer composition wherein
the inorganic particles and the polymer primary particles are
uniformly dispersed at the nano level regardless of the chemical
composition of the primary particles in the polymer dispersion. As
a result, other than the above described fluoropolymer dispersion,
it is possible to use many kinds of polymer dispersion, especially
those obtainable by emulsion polymerization.
Polymer dispersions may be made by other methods, such as by
melting the polymer and dispersing it, usually by mechanical
action, such as high shear mixing, in a medium, such as water,
usually with the aid of a surfactant (U.S. Pat. No. 2,995,533).
Alternatively, polymer may be dissolved in a solvent, this solution
dispersed in water with the aid of a surfactant, and then the
solvent removed by evaporation or by stripping, such as with
steam.
Examples of preferred non-fluorine-containing polymer dispersion
include polystyrene (PS), poly(methyl methacrylate) (PMMA),
poly(vinyl chloride) (PVC), polyisoprene, polybutadiene,
styrene/butadiene copolymer (SBR), acrylonitrile/butadiene
copolymer, methyl methacrylate/butadiene copolymer,
2-vinylpyridine/styrene/butadiene copolymer,
acrylonitrile/butadiene/styrene copolymer, poly(vinyl acetate)
(PVAc), and ethylene-vinyl acetate (EVAc).
The preferred particle diameter of the polymer primary particle in
the polymer dispersion depends on the particle diameter of the
inorganic particle in the colloid solution. For example, the
average polymer primary particle will generally be 50 to 500 nm,
and preferably, 70 to 300 nm.
The present invention uses colloidal solutions, also known as sols,
wherein inorganic particles are stably dispersed. Examples of the
inorganic particles of the sol include metals, including silica,
and metal compounds such as metal oxides, nitrides, zirconates
silicates, antimonates, titanates, and hydroxides. It is preferable
to use silicon oxide (SiO.sub.2), titanium oxide (TiO.sub.2),
zeolite, zirconium oxide (ZrO.sub.2), alumina (Al.sub.2O.sub.3),
and zinc antimonate (ZnSb.sub.2O.sub.6). These materials may be
used singly, or in combination of two or more. Examples of the
other suitable particles include silicon carbide (SiC), aluminum
nitride (AlN), silicon nitride (Si.sub.3N.sub.4), barium titanate
(BaTiO.sub.3), boron nitride, lead oxide, tin oxide, chrome oxide,
chromium hydroxide, cobalt titanate, cerium oxide, magnesium oxide,
cerium zirconate, calcium silicate, zirconium silicate, and
transition metals, including gold, silver, and copper. The only
limitation is that the particles be compatible with the components
of the dispersion, such as the aqueous medium, and the composition.
The most preferred inorganic particles are silicon oxide, titanium
oxide, aluminum oxide, and zinc antimonate.
It is preferable that the inorganic particle sol of the present
invention is stabilized in a liquid state by a variety of
electrolyte and organic additives. For example, colloidal silica
sol is a colloid solution wherein negatively-charged silicon oxide
nano particles are dispersed in water with silanol hydroxyl groups
present in the surface of the particles. The inorganic particles
are hydrophilic and are not treated to make them porous.
The particle diameter of the inorganic particles of the sol
normally averages 1 to 1000 nm, preferably, 5 to 500 nm, and more
preferably, 10 to 300 nm. Generally, for ease of preparation, sols
having inorganic particles with an average particle diameter of 5
to 500 nm are preferred. For best uniform dispersion of inorganic
particles at the nano level, it is especially preferable to use
inorganic particle sol having inorganic particles with an average
particle diameter of 10 to 300 nm. The colloids used in the present
invention, although containing inorganic particles, are considered
to be solution because the sol, generally at the usual low
concentration of the inorganic particles in the aqueous medium of
the sol, has the transparency of water, i.e. the particles are not
visible to the naked eye.
Any of the known methods for coagulating polymer dispersions may be
used. For example after the polymer dispersion is mixed with the
sol, the mixture may be subjected to strong shearing using a
stirring device thereby coagulating the particles (physical
coagulation). Another method of physical coagulation is the
freeze-thaw method. The mixture is cooled sufficiently to freeze
it. This destabilizes the dispersion so that on thawing, the
coagulate, which is the co-aggregate of the invention, separates
from the liquid. Also, there is a method wherein an electrolyte is
added to the mixture so that the stability of the mixture of
polymer dispersion or the inorganic particle colloid solution is
decreased thereby causing coagulation (chemical or electrolyte
coagulation). Among these methods, it is preferable to use the
chemical coagulation method wherein an electrolyte such as nitric
acid or inorganic salt is added to the mixture of polymer
dispersion and inorganic particle sol so that the stability is
decreased and the uniform mixture of the polymer primary particles
and the inorganic particles is solidified thereby obtaining
co-aggregate wherein inorganic particles and the primary polymer
particles are uniformly dispersed.
There are a variety of electrolytes used for the chemical
coagulation method depending on the type or ratio of the polymer
primary particles or inorganic particles in the mixed solution
before they are chemically coagulated. Examples of the electrolytes
which are used to chemically coagulate fluoropolymer primary
particles in a fluoropolymer aqueous dispersion include inorganic
or organic compounds such as aqueous HCl, H.sub.2SO.sub.4,
HNO.sub.3, H.sub.3PO.sub.4, Na.sub.2SO.sub.4, and MgCl.sub.2. Among
the above described compounds, it is preferable to use compounds
which can volatilize during the process of drying the co-aggregate
which is later conducted, such as HCl, HNO.sub.3 and also,
(NH.sub.4).sub.2CO.sub.3, and ammonium carbonate.
Furthermore, other than the above described electrolytes, it is
possible to use inorganic salts such alkali metal salt, alkaline
earth metal salt, and ammonium salt, of nitric acid, hydrohalic
acid, phosphoric acid, sulfuric acid, molybdic acid, sulfuric acid,
and preferably, potassium bromide, potassium nitrate, potassium
iodide (KI), ammonium molybdate, monobasic or dibasic sodium
phosphate, ammonium bromide (NH.sub.4Br), potassium chloride,
calcium chloride, copper chloride and calcium nitrate. The above
described electrolytes can be independently used or in combinations
of two or more. By repeatedly eluting the resulting co-aggregate
with pure water and then drying, it is possible to remove the
inorganic salt from the co-aggregate.
It is preferable to use 1 to 50 wt %, more preferably 1 to 30 wt %
of the above described electrolyte to the weight of the polymer,
more preferably, 1.5 to 30 wt %. It is also preferable to use 0.01
wt % to 30 wt %, more preferably 0.02 wt % to 10 wt % of the above
described electrolyte. Also, it is preferable to add the
electrolyte in the form of an aqueous solution to the mixed
solution of polymer dispersion and sol. If the amount of the
electrolyte is too small, coagulation occurs gradually and
incompletely. As a result, it may not be possible to solidify
rapidly enough to ensure that the uniformly mixed state of the
polymer primary particles and the inorganic particles will persist
through coagulation so as to ensure a co-aggregate wherein the
inorganic particles and primary polymer particles are uniformly
mixed.
The device for mixing and coagulating the particles wherein the
polymer dispersion is mixed with the inorganic particles and after
the polymer primary particles are uniformly mixed with the
inorganic particles, and an electrolyte or inorganic salt is added
to the mixture, is not limited to a specific type. However, it is
preferable to use a device which is equipped with a stirring means
such as propeller blades, turbine blades, paddle blades,
shell-shaped blades, horseshoe-shaped blades or spiral-shaped
blades, in which the stirring speed can be controlled. The device
should have a water-discharge means.
By adding the polymer dispersion, and inorganic particle sol to the
above described device and stirring, and then adding electrolyte
such as inorganic salt to the mixture and stirring, the colloid
particles or/and the inorganic particles are coagulated to create a
co-aggregate of polymer and inorganic particles which is then
separated from the aqueous medium. The aqueous medium is separated
from the co-aggregate and then said co-aggregate is washed with
water so as to reduce electrolyte residue to levels suitable for
the intended use of the co-aggregate. The separation step is the
recovery of the co-aggregate. After washing, the co-aggregate is
dried at a temperature below the melting point of the polymer and
the below the temperature at which thermal decomposition starts. It
is preferable that the temperature at which the co-aggregate is
dried is not so high that thermal degradation and thermal
decomposition of the polymer will occur, but high enough so that
volatile electrolyte and surfactant will be vaporized. Drying
conditions should include ventilation adequate to carry volatiles
away. The resulting dried co-aggregate is a powder wherein each
powder particle contains polymer primary particles and nano-sized
inorganic particles uniformly mixed.
The weight of inorganic particles in the mixture of the polymer
dispersion and inorganic particle colloid, depending on the
intended use of the polymer composition, is preferably 0.1 to 80 wt
%, more preferably, 0.3 to 50 wt %, and most preferably, 0.5 to 30
wt %, the balance being the polymer in the dispersion, to total
100% of the combined weights of polymer and inorganic particles.
Thus, the co-aggregate and granules, pellets and articles molded
therefrom that contain 0.1 to 80 wt % inorganic particles, will
contain 99.9 to 20 wt % of the polymer either as primary particles
or as polymer matrix obtained therefrom. In the nano polymer
composition mixture or polymer nano composite according to this
invention, wherein the inorganic particles and primary polymer
particles are uniformly dispersed at the nano level, when the
composite is heated sufficiently to melt the polymer component, the
interfacial area among the nano particles and the resulting polymer
matrix is significantly increased compared with that of the
conventional polymer compound mixture wherein filler is dispersed
at a micro level, that is, where the filler particles are greater
than about 1000 nm in size. Therefore, said polymer composition
mixture has the advantage that, even though the quantity of
inorganic particles added is smaller than that of the conventional
polymer composition mixture, the properties of the composite are
improved.
One of the characteristics of the polymer composition mixture of
the present invention wherein the polymer dispersion is mixed and
stirred with the sol where the inorganic particles are dispersed
and the polymer primary particles are uniformly mixed with the
inorganic particles, which mixture is then coagulated thereby
solidifying the uniformly mixed state of the polymer primary
particles and the inorganic particles, is that, since the inorganic
particles are uniformly dispersed at the nano level, after melting
sintering or compression molding (as in a hot press) of the polymer
component, the resulting composition viscosity and elasticity are
different from those of the conventional polymer mixtures wherein
inorganic particles are of a size of several thousands of
nanometers or greater.
Concentrated solutions of polymer or molten polymer are typically
non-Newtonian fluids and therefore their viscosities are dependent
upon shear rate. As shear rate increases, viscosity decreases, and
as shear rate decreases, viscosity increases. However, as the shear
rate tends towards zero, the viscosity approaches a constant value.
This limit value is called "zero shear rate viscosity". This is a
most important physical value which indicates the viscosity of a
polymer and is an exponential function of the polymer molecular
weight.
For example, the melt viscosity of a melt processible fluoropolymer
normally approaches a constant value as the shear rate is tends
towards zero and shows a Newtonian fluid-like behavior (FIG. 1
(curve A)). Also, the viscosity of the conventional polymer
composition mixture wherein fused silica with a particle diameter
of about 3000 nm is dispersed in the melt processible
fluoropolymer, is greater by a constant factor compared with the
melt processible fluoropolymer to which silica is not added. In
this case, when the shear rate is decreased, the viscosity
approaches a constant value showing a Newtonian fluid-like behavior
(FIG. 1(curve B)). However, in the case of the melt processible
fluoropolymer composition of the present invention wherein silica
with a particle diameter of about 66 nm is uniformly dispersed in
the melt processible fluoropolymer, when the shear rate is
decreased, the melt viscosity does not approach a constant value.
Instead, as the shear rate decreases, the viscosity further
increases (FIG. 1 (curve C) and (curve D)).
It is believed that the viscosity of the polymer composition
mixture of the present invention continues to increase when the
shear rate is decreased because the activity of the surface of the
nano particles is significantly increased and at the same time the
interfacial area among the nano particles and the polymer matrix is
significantly increased, and the distance among the nano particles
wherein nano particles are uniformly dispersed becomes shorter than
is the case for conventional polymer composition mixtures wherein
filler is dispersed at the micron level, i.e. >1000 nm. Silica
with a particle diameter of 70 nm completely nano-dispersed in the
polymer has a surface area of silica or alternatively, an
interfacial area with the polymer, of about 400 times greater than
the same weight of silica having a particle diameter of about 30000
nm.
The above described significant increase of the activity of the
surface of the nano particles and their surface area or interfacial
area is the characteristic of the polymer nano composite wherein
inorganic particles are dispersed at the nano level in the polymer
and is believed to be the reason why properties are improved even
though a smaller amount of the inorganic particles is used than
would be the case for a conventional polymer composition mixture.
For example, in the melt processible fluoropolymer composition
mixture wherein the inorganic nano particles of the present
invention are uniformly dispersed at the nano level, as the shear
rate decreases, the viscosity continues to increase. Therefore, the
composition is especially suitable for use as insulation for
electric wire. Such insulation, when exposed to high heat, as in a
fire, is less likely than conventional compositions to drip. This
is because of the viscosity-enhancing effect at low shear, such as
the shear force of gravity, of the nano particle filler. Reduced
dripping is desirable because drops of molten polymer are
hazardous, capable for example of causing smoke and propagating
fire.
Furthermore, the dispersive state of the nano particles in the
polymer can be directly observed by an electron scanning microscope
(SEM) or transmission electron microscope (TEM). It is necessary
use higher magnification with the nano particles than with
convention fillers, and therefore only the small local areas can be
observed. As a result, it is difficult to examine the dispersive
state of all the nano particles in a sample. However, by examining
changes of the viscosity as the shear rate is increased, it is also
possible to indirectly evaluate the dispersive state of the nano
particles.
In the case of the melt processible fluoropolymer composition which
is obtained by mixing and stirring the polymer dispersion with the
inorganic particle sol wherein the inorganic particles are
dispersed, and then coagulating to obtain co-aggregate, followed by
melting sintering or compression molding of the polymer component,
the melt viscosity is observed to vary with shear rate. The
increase in melt viscosity with decreasing shear is preferably
characterized by the ratio (V.sub.0.1/V.sub.1) of the melt
viscosity (V.sub.0.1) at 0.1 rad/sec to melt viscosity (V.sub.1) at
1 rad/sec, viscosity being measured at 340.degree. C. using the
parallel-plate mode of a dynamic viscosity and elasticity measuring
device. Depending on the relative amounts of the polymer primary
particles and the inorganic particles, and the particle diameter of
the inorganic particles, the ratio V.sub.0.1/V.sub.1 is preferably
1.4 or more, or preferably, 1.5 or more, or more preferably, 2.0 or
more.
In considering the ratio (D.sub.inorganic/D.sub.polymer) of the
diameter of the inorganic particle (D.sub.inorganic), to the
polymer primary particle (D.sub.polymer), when the mass of polymer
in the composition is greater than that of inorganic material, the
ratio (D.sub.inorganic/D.sub.polymer) is preferably about 0.1 or
greater, more preferably no less than about 0.2, and most
preferably no less than about 0.35. The ratio should not exceed
2.0. If the particle diameter of the inorganic particles is too
small relative to the diameter of the primary polymer particle, the
large polymer particles cannot cover or enclose (surround) the
small inorganic particles during coagulation, and the inorganic
particles thus tend to form their own large aggregates after
coagulation, rather than forming co-aggregate. In addition, if the
diameter of the inorganic particles is too large, the inorganic
particles tend to settle under the influence of gravity. This can
be a problem for the sol itself, and when the inorganic particle
sol is mixed with the polymer dispersion. The same ratios are
suitable for the case when the mass of polymer in the composition
is greater than that of inorganic material.
According to the present invention, the co-aggregate of particles
wherein the polymer primary particles and inorganic particles
obtained in the above described drying process, are uniformly
dispersed, can be melt-processed using known extrusion-molding
methods, injecting-molding methods, compression molding methods,
and transfer-molding methods. Such processing is preferably done
after the co-aggregate is pelletized, preferably in a melt
extruder. Of course, the co-aggregate if not pelletized can be
directly used in molding, or pelletized by compacting to improve
feeding to the molding machine hopper. Also, the co-aggregate of
the particles wherein the polymer primary particles and inorganic
particles obtained in the present invention are uniformly
dispersed, can be further granulated and used as the material for a
powder molding, powder coating and rotomolding, which includes
rotolining. One way in which such granulation can be achieved by
post-coagulation addition of a water-immiscible solvent, as
described in U.S. Pat. No. 4,675,380.
The co-aggregate, particularly the pelletized co-aggregate, may be
used as a "concentrate" to be blended with additional compatible
polymer. The resulting blend will have a lower concentration of
filler, such as silica, if a silica sol is used in making the
co-aggregate. By using co-aggregate as concentrate, it is not
necessary to make co-aggregate for each polymer composite needed.
The concentrate can be blended, preferably melt blended, if desired
by first dry blending, such as dry blending of pellets of the
composition with pellets of polymer, to give the desired
concentration of filler in the finished article.
Furthermore, when the co-aggregate is pelletized by using an
extruder, it is preferable to use a twin-screw extruder because of
its superior shearing force. Also, during the process of
pelletizing the co-aggregate in the extruder, it is possible to add
additive(s) or to blend in other polymer(s). The addition of an
additive can be done not only during the melt-extruding process but
also during the process where the above described polymer
dispersion and inorganic particle sol is mixed. Examples of
additives include glass fiber, carbon fiber, aramide fiber,
graphite, carbon black, mica, clay, fullerene, carbon nano tubes
and carbon nano fiber.
Because the particles are uniformly dispersed at the nano level in
the polymer, the final molded product can be used in a variety of
areas to improve properties. Examples of such molded product
include tubes, sheets, films, rods, fabrics, fibers, packing,
lining, seal rings, electric wire insulation, and film and print
substrate. The polymer composition in which the polymer itself is
transparent and in which the uniformly dispersed inorganic are
either small, or present in small amount, or both, is also
transparent. The inorganic nanoparticles are from 1 to 200 nm in
size and are resent in concentrations of from 0.1 to 40 wt % based
on the combined weights of polymer and inorganic particles. Such
compositions are useful for a variety of purposes such as a film
for anti-reflective coatings, anti-scratch film, optical fibers,
transparent film, transparent tubes and electric material.
Furthermore, since the particles are uniformly dispersed in the
polymer and the shear rate is significantly decreased, the zero
shear rate viscosity is significantly increased compared to the
case where the particles are not dispersed at the nano level.
Therefore, the present invention can be also used for polymer
products such as an electric wire insulation because of increased
resistance of the insulation to drip at high heat, such as in a
fire, because of the polymer high viscosity under the low shear of
gravity. This antidrip property is beneficial because it reduces
the danger of dripping of the molten polymer under fire
conditions.
EXAMPLES
Example 1
The present invention is described in detail in the following
Examples, which are not intended to be limiting.
(A. Measurement of the Properties)
(1) Melting Point (melting peak temperature)
A differential scanning calorimeter (Pyris 1 type DSC, made by
Perkin Elmer Co.) is used. About 10 mg of sample is weighed and
placed in an aluminum pan, which is then crimped. The crimped pan
is placed in the DSC and the temperature is increased from
150.degree. C. to 360.degree. C. at 10.degree. C./minute. The
melting peak temperature (Tm) is obtained from the melting curve
which is obtained in the above described process, being the maximum
of the endotherm.
(2) Melt Flow Rate (MFR)
Using a melt indexer (made by Toyo Seiki Co.) equipped with
corrosion resistant cylinder, die and piston which complies with
ASTM D-1238-95, 5 g of sample powder is put into the cylinder which
is kept at 372.+-.1.degree. C. and maintained for 5 minutes. After
that, the sample is extruded through a die orifice under 5 kg of
load (piston plus weight) and the extrusion rate (g/10 minute) is
the MFR. For PTFE, the molecular weight is too high to conduct a
normal melt-extruding operation, therefore the melt flow rate is
not measured.
(3) Particle Diameter
The particle diameter of the polymer primary particles in the
fluoropolymer dispersion and of the silica particles in the silica
sol is obtained as follows: the concentration of the fluoropolymer
dispersion or silica sol is diluted to 5 wt % by adding pure
(deionized or distilled) water, and dried. Then, the particles on
the surface of the dried samples are observed by an electron
microscope and the average particle diameters are obtained.
(4) Silica Dispersive State in the Polymer Matrix
A sheet having a thickness of about 200 .mu.m is made by
melt-compression-molding at 350.degree. C. fluoropolymer
composition of the invention. Sample pieces 10 mm.times.10 mm are
cut from three sections of the sheet. Using an optical microscope
(made by Nikon Co., OPTIPHOTO 2-POL), the dispersive state of the
particles, that is, whether or not there are aggregates of silica
nano particles of 1000 nm or more, is observed.
Samples in which the silica nano particles of 1000 nm or more are
observed, are placed in liquid nitrogen and fractured to expose
cross-sectional surfaces. The exposed surfaces of three samples are
observed by electron microscope to evaluate the dispersive state of
silica. The case where almost all of the silica is dispersed as
primary particles is expressed by .circleincircle.. The case where
only less than about 5% of the silica nano particles are aggregated
to greater than 1000 nm are observed is expressed by .smallcircle..
The case where a 20% or more of the silica nano particles are
aggregated to greater than 1000 nm is expressed by x.
(5) Zero Shear Rate Viscosity
Sample pieces with a diameter of 25 mm are made from
compression-molded (350.degree. C.) sheet about 1.5 mm thick. Using
a 25 mm-parallel plate in an ARES viscosity and elasticity
measuring device made by Rheometric Scientific Corporation (UK),
the melt viscosity is measured at 340.degree. C. over an
oscillation frequency (shear rate) of 100 to 0.1 rad/sec, and the
ratio (V.sub.0.1/V.sub.1) of the melt viscosity (V.sub.0.1) at 0.1
rad/sec to the melt viscosity (V.sub.1) at 1 rad/sec is
calculated.
(6) Storage Elastic Modulus
Sample pieces of 12 mm.times.45 mm.times.1.5 mm are made from a
compression-molded (350.degree. C.) sheet of about 1.5 mm thick.
Using an ARES viscosity and elasticity measuring device made by
Rheometric Scientific Corporation, the storage elastic modulus is
measured in torsion mode at 1 Hz from -40.degree. C. to 200.degree.
C. at a heating rate of 5.degree. C./minute.
(B. Materials)
The starting materials used in the examples of the present
invention and the comparative examples are described:
(1) PFA Emulsion
Made by DuPont Mitsui Fluorochemical Co. PFA aqueous dispersion is
obtained by emulsion polymerization. Polymer solids: 30 wt %;
average particle diameter of the PFA primary particles: 200 nm; pH
9; melting point: 309.degree. C.; and melt flow rate: 2 g/10
min.
(2) Pelletized PFA
(melting point: 309.degree. C.; and melt flow rate: 2 g/10
minutes)
(3) PTFE Emulsion
(polymer solids: 50 wt %; average particle diameter of the primary
particles: 210 nm; pH 9; and melting point: 326.degree. C.)
(4) Silica Sol
(a) Made by Nissan Chemical Corporation, Snowtex MP2040
(silica: 40 wt %; silica primary particle diameter: 190 nm; and pH
9.5)
(b) Made by Nissan Chemical Corporation, Snowtex MP1040
(silica: 40 wt %; silica primary particle diameter: 110 nm; and pH
9.5)
(c) Made by Fuso Chemical Corporation, PL-7
(silica: 23 wt %; silica primary particle diameter: 70 nm; and pH
7.4)
(d) Made by Fuso Chemical Corporation, PL-3
(silica: 20 wt %; silica primary particle diameter: 35 nm; and pH
7.2)
(5) Fused Silica
Made by Denki Kagaku Kogyo, FB-74 (silica average particle
diameter: 32000 nm)
Example 1
Silica sol, 33 g, made by Nissan Chemical Co. (MP-2040), and 1000 g
of pure water are placed in a beaker (2 L) which is stirred for 20
minutes at 200 rpm using a stirrer with four-blade down draft-type
propeller. Then, 853 g of emulsion polymerized PFA aqueous
dispersion is added to the mixture so that the silica content
becomes 5 wt % of the combined weight of polymer and silica. After
the mixture is stirred for another 20 minutes, 9 ml of 60% aqueous
nitric acid is added to the mixture. Said mixture is stirred again
until it gels and fluoropolymer primary particles and silica nano
particles are coagulated. The coagulated co-aggregate is further
stirred for 5 minutes at 350 rpm and then separated from the
aqueous medium. After that, the co-aggregate is dried at
150.degree. C. for 10 hours thereby obtaining an co-aggregate in a
dried-powder form. The dried co-aggregate powder is compression
molded at 350.degree. C., giving a sheet having a thickness of
about 1.5 mm. Elasticity and viscosity are measured and the sample
is observed by using an optical and electron microscopes. The
results are summarized in Table 1 and in FIG. 3.
Example 2
Dried co-aggregate powder is made by the same procedure as that of
Example 1 except that the amount of the silica sol and the PFA
aqueous dispersion is adjusted so that the silica content is 15 wt
%. The dried co-aggregate powder is compression molded at
350.degree. C. and, by using the resulting sheet having a thickness
of about 1.5 mm, the elasticity and viscosity are measured. Results
are summarized in Table 1.
Example 3
Dried co-aggregate powder is made by the same procedure as that of
Example 1 except that the amount of the silica sol and the PFA
aqueous dispersion is adjusted so that the silica content is 20 wt
%. The dried co-aggregate powder is compression molded at
350.degree. C. and, using the resulting sheet having a thickness of
about 1.5 mm, the elasticity and viscosity are measured and the
sample is observed by using optical and electron microscopes. The
results are summarized in Table 1 and FIG. 1 (curve C).
Example 4
Dried co-aggregate powder is made by the same procedure as that of
Example 1 except that PL-7 is used as the silica sol instead of
MP-2040 and the silica content is 10 wt %. The dried co-aggregate
powder is compression molded at 350.degree. C. and, using the
resulting sheet having a thickness of about 1.5 mm, the elasticity
and viscosity are measured and the sample is observed using optical
and electron microscopes. The results are summarized in Table 1 and
FIGS. 2 (curve E) and 4. Also, to observe the dispersed state of
PFA particle and silica particle mixture after coagulation, the
dried co-aggregate powder is further dried at 295.degree. C. for 12
hrs before being subjected to observation by electron microscopy.
The results are shown in FIG. 5.
Example 5
Dried co-aggregate powder is made by the same procedure as that of
Example 4 except that the silica content is 20 wt % (PL-7). The
dried co-aggregate powder is compression molded at 350.degree. C.
and, using the resulting sheet having a thickness of about 1.5 mm,
the elasticity and viscosity are measured and the sample is
observed by using optical and electron microscopes. The results are
summarized in Table 1 and FIGS. 1 (curve D) and 2 (curve D).
Example 6
Dried co-aggregate powder is made by the same procedure as that of
Example 1 except that PL-3 is used as the silica sol instead of
MP-2040 and the silica content is 20 wt %. The dried co-aggregate
powder is compression molded at 350.degree. C. and, using the
resulting sheet having a thickness of about 1.5 mm, the elasticity
and viscosity are measured and the sample is observed by using
optical and electron microscopes. The results are summarized in
Table 1.
Example 7
This Example uses PTFE that cannot be melt-processed. The dried
co-aggregate powder is made by the same procedure as that of
Example 1 except that PTFE aqueous dispersion is used instead of
PFA dispersion. The PTFE dispersion is diluted with pure water to a
solids concentration of 30 wt %. The silica content is 5 wt %. The
melt viscosity of the PTFE is extremely high, so the viscosity is
not measured. The dried co-aggregate powder is compression molded
at 350.degree. and, using the resulting sheet having a thickness of
about 1.5 mm, the elasticity is measured and the sample is observed
by using optical and electron microscopes. Because of the high
viscosity of PTFE, viscosity is not measured. The results are
summarized in Table 1 and FIG. 6.
Comparative Example 1
Fused silica with an average particle diameter of 32000 nm is
melt-mixed with pelletized PFA pellet using an R-60 melt-mixer
(made by Toyo Seiki Co.) at 340.degree. C. at 100 rpm for 5
minutes. This process gives a conventional composition wherein
silica with an average particle diameter of 32000 nm is dispersed
in the melt processible fluoropolymer is obtained. The silica
content is 20 wt %. The resulting sample is compression molded at
350.degree. C. and, using the resulting sheet having a thickness of
about 1.5 mm, the elasticity and viscosity are measured and the
sample is observed by using an optical and electron microscopes.
The results are summarized in Table 1 and FIG. 1 (curve B).
Comparative Example 2
In this example film is made by directly coating the solution of
fluoropolymer aqueous dispersion mixed with silica sol on a
substrate. The mixed dispersion and sol is not coagulated. Silica
sol, 33 g, made by Nissan Chemical Co. (MP-1040) and 1000 g of pure
water are placed into a beaker (2 L) which is stirred for 20
minutes at 200 rpm by using a down flow-type propeller four-blade
stirring machine. PFA aqueous dispersion, 853 g, made by emulsion
polymerization, is added to the mixture so that a mixture having
weight ratio of PFA to silica of 95/5 is obtained. This mixture is
then stirred for another 20 min. As a result, a solution of
fluoropolymer dispersion mixed with silica sol is obtained. The
silica content is 5 wt %. The mixed solution is directly
spray-coated on an aluminum plate which is dried at 120.degree. C.
for 30 minutes and sintered at 350.degree. C. for 15 minutes,
thereby obtaining a coated product with a coating thickness of
about 50 .mu.m. The surface of the coating product is observed by
optical and electron microscopes and the results are summarized in
Table 1 and FIG. 7.
Comparative Example 3
In this example the mixed solution of the fluoropolymer dispersion
and silica sol is dried without being coagulated. Using the same
method as in Example 4 except that MP-1040 is used as the silica
sol instead of MP2040, a mixed solution of the fluoropolymer
aqueous dispersion and silica sol wherein the silica content is 10
wt %, is obtained. The mixed solution is dried at 80.degree. C. for
12 hours thereby creating dried powder. The obtained dried
co-aggregate powder is compression molded at 350.degree. C. and,
using the resulting sheet having a thickness of about 1.5 mm, the
elasticity and viscosity are measured and the sample is observed by
using optical and electron microscopes. The results are summarized
in Table 1 and FIG. 8.
Reference Example 1
The properties of the melt processible fluoropolymer itself, that
is without added silica or other filler, are summarized in Table 1
and FIGS. 1 (curve A) and 2 (curve A).
Summary of Results from Examples
In Examples 1 to 3, the silica nano particles are completely
nano-dispersed in the melt processible fluoropolymer matrix. Due to
the nano-dispersed silica, the viscosity ratio (V.sub.0.1/V.sub.1)
is higher than that of the pure melt processible fluoropolymer
(Reference Example 1). As the silica content is increased, the
viscosity ratio (V.sub.0.1/V.sub.1) is increased. Also, as the
amount of silica is increased, the storage elastic modulus is
increased.
In Examples 4 and 5, the silica nano particles are completely
nano-dispersed in the melt processible fluoropolymer matrix. Also,
comparing the samples with silica is 20 wt % silica content, the
viscosity ratio (V.sub.0.1/V.sub.1) is greater for Example 5 where
the particle diameter of silica is smaller than that of Example 3.
Especially, in Example 5, aggregates of silica particles are not
observed on the surface of the mixture wherein the PFA primary
particles (average particle diameter: about 200 nm) and the silica
particles (average particle diameter: about 70 nm) are coagulated
before the dried co-aggregate powder is compression molded.
It is seen in Example 6, even in the case where the particle
diameter of silica is 35 nm, the silica nano particles are
completely nano-dispersed in the melt processible fluoropolymer
matrix. Also, the aggregate made of silica nano particles with a
size of 1000 nm or more is not observed by an optical microscope.
However, a few aggregates with a size of about several hundreds nm
made of silica nano particles with a particle diameter of 35 nm are
observed by an electron microscope at 20000-fold magnification.
Furthermore, the viscosity ratio (V.sub.0.1/V.sub.1) is almost the
same as that of Example 5 wherein the particle diameter is 70 nm
and the silica content is also 20%.
TABLE-US-00001 TABLE 1 Fluoropolymer Silica volume (%) Primary
particle PFA PTFE Weight (%) Type diameter (nm)
D.sub.inorganic/D.sub.polymer Example 1 95 -- 5 MP-2040 190 0.95
Example 2 85 -- 15 MP-2040 190 0.95 Example 3 80 -- 20 MP-2040 190
0.95 Example 4 90 -- 10 PL-7 70 0.35 Example 5 80 -- 20 PL-7 70
0.35 Example 6 80 -- 20 PL-3 35 0.175 Example 7 -- 95 5 MP-2040 190
0.95 Comparative 80 -- 20 FB-74 (32000) -- Example 1 Comparative 95
-- 5 MP-1040 110 0.55 Example 2 Comparative 90 -- 10 MP-1040 110
0.55 Example 3 Referential 100 -- -- -- -- -- Example 1 Properties
of the composition Silica Storage elastic modulus (Pa) V.sub.0.1
V.sub.1 dispersive 25.degree. C. 100.degree. C. 200.degree. C. (Pa
s) (Pa s) V.sub.0.1/V.sub.1 state Example 1 3.10E+08 1.00E+08
4.00E+07 31020 20110 1.54 .circleincircle. Example 2 4.50E+08
1.60E+08 6.90E+07 79837 39388 2.03 .circleincircle. Example 3
6.10E+08 2.50E+08 1.10E+08 195070 62233 3.13 .circleincircle.
Example 4 4.20E+08 1.40E+08 6.00E+07 80544 36946 2.18
.circleincircle. Example 5 6.40E+08 2.80E+08 1.40E+08 455840 91241
5.00 .largecircle. Example 6 6.60E+08 3.20E+08 1.60E+08 1063700
156200 6.81 .largecircle. Example 7 Not measured Not measured
.circleincircle. Comparative 3.80E+08 1.20E+08 4.30E+07 39331 34553
1.14 X Example 1 Comparative Not measured Not measured X Example 2
Comparative 3.30E+08 1.20E+08 4.50E+07 24048 17813 1.35 X Example 3
Reference 2.40E+08 7.30E+07 2.80E+07 20864 18090 1.15 na Example 1
Note: "na" means not applicable
In Examples 3, 5 and 6, when the silica content is 20 wt %, as the
particle diameter of silica decreases, the storage elastic modulus
increases. In Example 7, since the melt viscosity of PTFE is very
high, it is not practical to mix additives or nanoparticles with
PTFE by melt-mixing. The present invention offers a way to
uniformly disperse silica nanoparticles in the PTFE matrix.
Comparative Example 1 is the conventional polymer composition
wherein silica with an average particle diameter of 32000 nm is
dispersed in the melt processible fluoropolymer. The viscosity
ratio (V.sub.0.1/V.sub.1) is almost the same as that of the melt
processible fluoropolymer without silica. This is because silica is
not nano-dispersed and is not effective in changing the viscosity
ratio from that of the melt processible fluoropolymer alone.
In Comparative Example 2 a film is made by directly coating a
substrate with the solution resulting from mixing of fluoropolymer
dispersion with silica sol. The co-aggregation step, e.g.
coagulation with electrolyte, is omitted. Because there is no
co-aggregation during the drying of the mixed solution, the
fluoropolymer primary particles and silica nano particles separate
and cluster and the silica nano particles similarly cluster to a
size of several micrometers, seen on the surface of the film after
sintering.
The transparency of the polymer compositions of the Examples were
determined, using pieces 50 mm.times.50 mm made from
compression-molded (350.degree. C.) sheet about 1 mm thick. Using a
Haze-meter NDH2000 (Nippon Denshoku, Japan) equipped with a halogen
lamp D65, the optical transmittance of the samples were measured.
The averaged values of optical transmittance were calculated from
results of three sample pieces. Transmittances of 50% or greater
appear transparent to the naked eye.
Example 1 PFA with 5 wt % 190 nm silica had a transmittance of 50%.
Examples 2 and 3 with 15 and 20 wt % respectively of 190 nm silica
had transmittance of 30 and 20%, showing that 190 nm particles
affect transparency only at higher loadings. Examples 4 and 5 are
PFA with 70 nm silica at loadings of 10 and 20 wt % respectively
have high transmittance of 72 and 70%, showing that the smaller
particles can be used at higher loadings without interfering with
transparency. Example 6 is PFA with a 20 wt % loading of 35 nm
silica and has 70% transmittance. In Example 7, PTFE with 5%
loading of 190 nm silica, i.e. the same loading of the same sized
silica as Example 1, has low transmittance, 10%. This is the effect
of the PTFE polymer, which being highly crystalline, has low
transparency, the crystals scattering light.
In conclusion, it is found that without co-aggregating it is not
possible to nano-disperse silica. To nano-disperse silica, it is
necessary to coagulate the mixed solution of the fluoropolymer
aqueous dispersion and silica sol and solidify the uniformly mixed
state of the polymer primary particles and inorganic particle. In
Comparative Example 3, the mixed solution of the fluoropolymer
aqueous dispersion and silica sol is not coagulated but rather
directly dried. The result is the clustering of the silica nano
particles (aggregation of silica particles with each other).
According to the present invention, polymer dispersion wherein
polymer primary particles are surrounded by a surfactant and stably
dispersed in the solvent, such as by emulsion polymerization, is
mixed and stirred with a colloid solution and said inorganic
particles are stably dispersed by a repulsive force among the
inorganic particles. It is not necessary to surface-treat the
inorganic particles. After the polymer primary particles and
inorganic particles are uniformly mixed, then coagulated by strong
shearing using a mixer, by adding an electrolyte, or by freezing
the dispersion. As a result, the stability of the polymer
dispersion and that of the inorganic particle colloid solution is
decreased thereby coagulating the particles. As a result, the
uniformly mixed state of the polymer primary particles and the
inorganic particles is solidified. Then, by separating the
co-aggregated particles from the solvent and drying, it is possible
to obtain the polymer composition wherein the inorganic particles
are intimately mixed at the nano level with the polymer particles.
Therefore, the present invention can be used for a variety of
fields which benefit when the inorganic particles are uniformly
dispersed at the nano level in polymer.
Furthermore, when the particles are uniformly dispersed in the
molten polymer and the shear rate is significantly decreased, the
zero shear rate viscosity is significantly increased compared with
the case where the inorganic particles are not dispersed at the
nano level. Therefore, the present invention can be also used for a
polymer product such as an electric wire insulation because of
increased resistance of the insulation to drip at high heat, such
as in a fire, because of the polymer's high viscosity under the low
shear of gravity. This antidrip property is beneficial because it
reduces the danger of dripping molten polymer under fire
conditions.
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